Method and apparatus for reducing matrix density effects on porosity measurements during epithermal neutron porosity well logging

Abstract

The porosity of the subsurface earth formation surrounding a borehole is investigated by measuring the populations of epithermal neutrons at detector locations longitudinally spaced from a neutron source. During this measuring process, a detector is located at the matrix density neutral (MDN) distance from the neutron source at which the matrix density effects of the earth formation are significantly reduced. The porosity measurements are determined from neutron counts detected at both the MDN location and other detector locations to derive formation porosity measurements which have reduced lithology and matrix density effects. Because of these reduced effects, the derived formation porosity values do not need to be substantially corrected to account for lithology and matrix density effects. In addition, the formation lithology and matrix density may be obtained from measurements taken at the MDN location and other detector locations.

We claim: 
     
       1. A pulsed neutron porosity logging method comprising the steps of: (a) repetitively irradiating a subsurface formation with bursts of neutrons from a neutron source;   (b) measuring, by means of a first detector spaced from the source at a first given distance, a count of epithermal neutrons indicative of epithermal neutrons within the irradiated subsurface formation as a measure of matrix density neutral porosity of subsurface formation; and   (c) combining the measurements of epithermal neutron counts from step (b) for a succession of the bursts to provide an indication of subsurface formation porosity.   
     
     
       2. The method of claim 1 further comprising before step (c) the step of normalizing the measurements of the epithermal neutron counts. 
     
     
       3. The method of claim 1 further comprising the step of: (d) determining the die-away rate of epithermal neutrons measured by the first detector in step (b) to provide another indication of subsurface formation porosity.   
     
     
       4. The method of claim 3 further comprising the step of comparing the subsurface formation porosity indicators derived from the measurements of epithermal neutron counts from steps (c) and (d) respectively. 
     
     
       5. The method of claim 1 further comprising the step of deriving an indication of the variations in the intensity of the output of the neutron source and compensating the measured epithermal counts for the variations. 
     
     
       6. The method of claim 1 further comprising the steps of: (d) measuring, by means of a second detector spaced from the source at a second given distance, a count of epithermal neutrons during a die-away spectrum for the epithermal neutrons within the irradiated subsurface formation;   (e) combining the measurements of epithermal neutron counts from step (d) for a succession of the bursts to provide another indication of subsurface formation porosity; and   (f) comparing the measurements of epithermal neutron counts from steps (c) and (e) to provide an indication of subsurface formation matrix density.   
     
     
       7. The method of claim 1 wherein the first given distance is approximately 30 centimeters. 
     
     
       8. The method of claim 6 wherein the first given distance is approximately 30 centimeters and wherein the second distance is approximately 64 centimeters. 
     
     
       9. The method of claim 6 further comprising the steps of: (g) measuring, by means of a third detector spaced from the source at a third given distance, a count of epithermal neutrons during a die-away spectrum within the irradiated subsurface formation;   (h) combining the measurements of epithermal neutron counts from step (g) for a succession of the bursts;   (i) deriving the ratio of combined counts from steps (h) and (e) respectively to provide an indication of subsurface porosity.   
     
     
       10. The method of claim 9 wherein the third detector is located between the neutron source and the first detector. 
     
     
       11. A method for investigating the porosity of a subsurface earth formation surrounding a borehole comprising the steps of: (a) positioning at least one detector at a first location in the borehole, the location being a longitudinal distance from a neutron source;   (b) positioning at least one detector at a second location in the borehole from the neutron source such that the distance between the second detector location and the source is the matrix density neutral distance;   (c) repetitively irradiating the borehole and earth formation with high energy neutrons from the neutron source, which neutrons interact with nuclei of the materials in the borehole and the formation to produce therein populations of epithermal neutrons;   (d) detecting the populations of epithermal neutrons at the first and second detector locations; and   (e) determining the porosity of the subsurface earth formation from the detected neutron populations at the detector locations.   
     
     
       12. The method of claim 11 wherein the distance from the second detector location to the neutron source is in the range of 28 to 33 centimeters. 
     
     
       13. The method of claim 11 wherein one of the detectors is an array of epithermal detectors. 
     
     
       14. The method of claim 11 further comprising before step (e) the step of generating neutron count signals indicative of the magnitudes of the detected epithermal neutron populations at the respective first and second detector locations. 
     
     
       15. The method of claim 11 wherein the porosity determination in step (e) comprises the steps of: generating neutron count signals, the signals being indicative of the magnitudes of detected epithermal neutron populations at the respective first and second detector locations; and   forming a ratio of the two epithermal neutron population count signals and deriving a signal representative thereof:   converting the ratio signal according to a predetermined relationship to derive the porosity measurement.   
     
     
       16. A method for investigating the porosity of a subsurface earth formation surrounding a borehole comprising the steps of: (a) positioning at least one detector at a first location in the borehole that is a longitudinal distance from a neutron source;   (b) positioning at least one detector at a second location in the borehole from the neutron source such that the distance between the second detector and the source is the matrix density neutral distance;   (c) repetitively irradiating the borehole and earth formation with discrete bursts of high energy neutrons from the neutron source, which neutrons interact with nuclei of the materials in the borehole and the formation to produce therein populations of epithermal neutrons;   (d) detecting the populations of epithermal neutrons at the first and second detector locations in the borehole;   (e) generating count signals indicative of the magnitudes of the detected epithermal neutron populations at the respective first and second detector locations; and   (f) detecting neutron source output intensity;   (g) deriving an indication of the variations in the intensity of the output of the neutron source and compensating the measured epithermal counts for the variations;   (h) deriving from the first and second count signals a measurement signal representative of the porosity of the formation surrounding the borehole the measurement having reduced effects from the lithology and matrix density of the formation.   
     
     
       17. The method of claim 16 wherein the porosity measurement of step (f) is derived by forming a ratio of the two epithermal neutron population count signals and deriving a signal representative thereof; and   converting the ratio signal according to a predetermined relationship to derive the porosity measurement.   
     
     
       18. The method of claim 16 wherein the neutron source output intensity is detected using a neutron monitor. 
     
     
       19. A method for investigating the porosity of a subsurface earth formation surrounding a borehole comprising: (a) repetitively irradiating the borehole and earth formation with discrete bursts of high energy neutrons from a neutron source, which neutrons interact with nuclei of the materials in the borehole and the formation to produce therein populations of epithermal neutrons;   (b) detecting the populations of epithermal neutrons at first and second locations in the borehole spaced apart longitudinally by different distances from the neutron source, the second location being at a distance from the source that is approximately the matrix density neutral distance;   (c) generating count signals indicative of the magnitudes of the detected epithermal neutron populations at the respective first and second locations;   (d) detecting the die-away of the epithermal neutron populations following the neutron bursts at least at one location in the borehole and generating signals representative thereof:   (e) deriving from said die-away signals a signal indicative of the slowing down time of epithermal neurons in the formation at said at least one location; and   (f) deriving from the first and second count signals and the slowing down time signal a measurement signal representative of the porosity of the formation surrounding the borehole inherently compensated for the effects of logging tool standoff on the responses of the logging tool.   
     
     
       20. A method of claim 19 wherein said porosity measurement deriving step (f) comprises combining, according to a predetermined relationship, a signal representative of a ratio of the first and second location count signals and the epithermal slowing down time signal to derive said standoff-compensated measurement signal of the formation porosity. 
     
     
       21. A method of claim 19 wherein said porosity measurement deriving step (f) comprises: deriving from said first and second count signals a first measurement signal representative of formation porosity and from said epithermal slowing down time signal a second measurement signal representative of formation porosity; and   combining said first and second porosity measurement signals according to a predetermined relationship to derive a correction factor signal indicative of the effect of tool standoff on said first porosity measurement signal.   
     
     
       22. A method of claim 21 wherein said porosity measurement deriving step further comprises combining said correction factor signal with said first porosity measurement signal to derive said standoff-compensated porosity measurement signal. 
     
     
       23. A method for investigating the porosity of a subsurface formation surrounding a borehole, comprising: (a) repetitively irradiating the well borehole and surrounding earth formation with bursts of high energy neutrons from a neutron source, which neutrons interact with nuclei of the materials in the borehole and the formation to produce therein populations of epithermal neutrons;   (b) measuring the magnitude of the epithermal neutron populations at least at two locations spaced at different distances along the borehole from the neutron source and one location being at a distance from the source that is approximately the distance at which the matrix density effects of the formation are neutral and generating respective count signals representative thereof;   (c) measuring the die-away of the epithermal neutron population between bursts at least at one location along the borehole and generating signals representative thereof;   (d) deriving from said at least two epithermal neutron population count signals a first measurement of formation porosity as a function of the slowing down length of epithermal neutrons in the earth formation;   (e) deriving form said epithermal neutron population decay signals a second measurement of formation porosity as a function of the slowing down time of epithermal neutrons in the earth formation; and   (f) combining said first and second porosity measurements to derive a standoff-compensated measurement of formation porosity.   
     
     
       24. A method of claim 23 wherein said first porosity measurement is derived by: forming a ratio of said at least two epithermal neutron population count signals and deriving a signal representative thereof; and   converting said ratio signal according to a predetermined relationship to derive said first porosity measurement.   
     
     
       25. A method of claim 24 wherein said second porosity measurement is derived by: die-away signals a measurement of the epithermal neutron slowing down time of the earth formation and generating a signal representative thereof; and   converting said epithermal slowing down time measurement signal according to a predetermined relationship to derive said second porosity measurement.   
     
     
       26. A method for determining the matrix density of a subsurface earth formation surrounding a borehole comprising the steps of: (a) detecting the population of epithermal neutrons at first, second and third detector locations in the borehole spaced apart longitudinally by different distances from the neutron source, said second detector location being between the first and third detector locations and at a location from the neutron source such that the distance between the second detector location and the source is the matrix density neutral distance;   (b) deriving the porosity of the subsurface formation by converting the ratio of the neutron population count signals of the first and second detector locations into a porosity value according to a predetermined relationship;   (c) deriving the porosity of the subsurface formation by converting the ratio of the neutron population count signals of the first and third detector locations into a porosity value according to a predetermined relationship;   (d) plotting curves of the two porosity responses for various subsurface formations;   (e) determining on the plot of step d the point where crossplots of the formation porosities of steps b and c would intersect; and   (f) interpolating the formation matrix density from the intersection point determined in step e.   
     
     
       27. The method of claim 26 wherein the crossplots of step (e) are determined by projecting straight lines of the two porosity values from different axes such that the lines intersect at a point that is the formation matrix density. 
     
     
       28. A method for investigating the hydrogen index of a subsurface earth formation surrounding a borehole comprising the steps of: (a) positioning at least one detector at a first location in the borehole, the location being a longitudinal distance from a neutron source;   (b) positioning at least one detector at a second location in the borehole from the neutron source such that the distance between the second detector location and the source is the matrix density neutral distance of the formation;   (c) repetitively irradiating the borehole and earth formation with high energy neutrons from the neutron source, which neutrons interact with nuclei of the materials in the borehole and the formation to produce therein populations of epithermal neutrons;   (d) detecting the populations of epithermal neutrons at the first and second detector locations; and   (e) determining the hydrogen index of the formation from a ratio of the neutron population detected at the first detector location to the neutron population detected at the second detector location.   
     
     
       29. An apparatus for epithermal neutron logging of subsurface formations surrounded by a borehole comprising: (a) an elongated support;   (b) a pulsed source of neutrons carried on the support;   (c) a first epithermal neutron detector carried on the support at a first distance from the source, the first distance being selected to provide epithermal counts as a measure of matrix density neutral porosity of subsurface formations;   (d) a means for repetitively pulsing the source so as to generate bursts of fast neutrons from the source into the subsurface formations;   (e) a means employing the detector for measuring a count of epithermal neutrons for the epithermal neutrons within the irradiated subsurface formation between each of the bursts of neutrons; and   (f) a means for combining the measurements of epithermal neutron counts from the first detector for a succession of the bursts to provide a first indication of subsurface formation porosity.   
     
     
       30. The apparatus of claim 29 wherein the means for measuring the count of epithermal neutrons includes means for time integrating the epithermal neutron counts. 
     
     
       31. The apparatus of claim 29 further comprising a means for determining the die-away rate of epithermal neutrons measured by the first detector to provide a second indication of subsurface formation porosity. 
     
     
       32. The apparatus of claim 31 further comprising a means for comparing the subsurface formation porosity indicators derived from the measurements of epithermal neutron counts from the first and second porosity indicators respectively. 
     
     
       33. The apparatus of claim 31 further comprising a means for deriving an indication of the variations in the intensity of the output of the neutron source and compensating the measured epithermal counts for the variations. 
     
     
       34. The apparatus of claim 29 further comprising: (g) a second epithermal neutron detector spaced from the source of a second given distance;   (h) a means employing the second detector for measuring a count of epithermal neutrons during a die-away spectrum for the epithermal neutrons within the irradiated subsurface formation between each of the bursts of fast neutrons;   (i) a means for combining the measurements of epithermal neutron counts from for a succession of the bursts to provide another indication of subsurface formation porosity; and   (j) a means for comparing the measurements of epithermal neutron counts from the first and second detectors to provide an indication of subsurface formation matrix density.   
     
     
       35. The apparatus of claim 29 wherein the first given distance is approximately 30 centimeters. 
     
     
       36. The apparatus of claim 33 wherein the first given distance is approximately 30 centimeters. 
     
     
       37. The apparatus of claim 34 wherein the first given distance is approximately 30 centimeters and wherein the second distance is approximately 64 centimeters. 
     
     
       38. The apparatus of claim 34 further comprising: (k) third detector spaced from the source at a third given distance;   (l) a means, employing the third detector for measuring a count of epithermal neutrons within the irradiated subsurface formation between each of the bursts of fast neutrons;   (m) a means for combining the measurements of epithermal neutron counts from the third detector for a succession of the bursts; and   (n) a means for deriving the ratio of combined counts from steps the second and third detectors respectively to provide an indication of subsurface porosity.   
     
     
       39. The apparatus of claims 29 or 34 or 38 further comprising means for deriving a record of the porosity indications as a function of the depth of the detector in the borehole. 
     
     
       40. An apparatus for investigating the porosity of a subsurface earth formation surrounding a borehole comprising: (a) means for repetitively irradiating the borehole and earth formation with discrete bursts of high energy neutrons form a neutron source, which neutrons interact with nuclei of the materials in the borehole and the formation to produce therein populations of epithermal neutrons;   (b) means for detecting the populations of epithermal neutrons at first and second locations in the borehole spaced apart longitudinally by different distances from the neutron source with one detector locations being at a distance from the source at which the matrix density effects of the formation are neutral;   (c) means for generating count signals indicative of the magnitudes of the detected epithermal neutron populations at the respective first and second locations;   (d) means for detecting the die-away of the epithermal neutron populations following the neutron bursts at least at one location in the borehole and generating signals representative thereof;   (e) means for deriving from said die-away signals a signal indicative of the slowing down time of epithermal neutrons in the formation at said at least one location; and   (f) means for deriving from the first and second count signals and the slowing down time signal a measurement signal representative of the porosity of the formation surrounding the borehole compensated for the effects of logging tool standoff on the responses of the logging tool.   
     
     
       41. The apparatus of claim 40 wherein said porosity-measurement deriving means comprises means for combining, according to a predetermined relationship, a signal representative of a ratio of the first and second location count signals and the epithermal slowing down time signal to derive said standoff-compensated measurement signal of the formation porosity. 
     
     
       42. The apparatus of claim 40 wherein said porosity-measurement deriving means comprises: means for deriving from said first and second count signals a first measurement signal representative of formation porosity and from said epithermal slowing down time signal a second measurement signal representative of formation porosity; and   means for combining said first and second porosity measurement signals according to a predetermined relationship to derive a correction factor signal indicative of the effect of tool standoff on said first porosity measurement signal.   
     
     
       43. The apparatus of claim 40 wherein said porosity-measurement deriving means further comprises means for combining said correction factor signal with aid first porosity measurement signal to derive said standoff-compensated porosity measurement signal. 
     
     
       44. An apparatus for investigating the porosity of a subsurface earth formation surrounding a borehole, comprising: (a) means for repetitively irradiating the well borehole and surrounding earth formation with bursts of high energy neutrons from a neuron source, which neutrons interact with nuclei of the materials in the borehole and formation to produce therein populations of epithermal neutrons;   (b) means for measuring the magnitude of the epithermal neutron populations at least at two locations spaced at different distances along the borehole from the neutron source one detector location being at a distance from the source at which the matrix density effects of the formation are neutral and generating respective count signals representative thereof;   (c) means for measuring the die-away of the epithermal neutron population between bursts at least a one location along the borehole and generating signals representative thereof;   (d) means for deriving from said at least two epithermal neutron population count signals a first measurement of formation porosity as a function of the slowing down length of epithermal neutrons in the earth formation;   (e) means for deriving from said epithermal neutron population die-away signals a second measurement of formation porosity as a function of the slowing down time of epithermal neutrons in the earth formation; and   (f) means for combining said first and second porosity measurements to derive a standoff-compensated measurement of formation porosity.   
     
     
       45. The apparatus of claim 44 wherein said means for deriving said first porosity measurement comprises: means for forming a ratio of said at least two epithermal neuron population count signals and deriving a signal representative thereof; and   means for converting said ratio signal according to a predetermined relationship to derive said first porosity measurement.   
     
     
       46. The apparatus of claim 44 wherein said means for deriving said second porosity measurement comprises: means for deriving from said epithermal neutron population die-away signals a measurement of the epithermal neutron slowing down time of the earth formation and generating a signal representative thereof; and   means for converting said epithermal slowing down time measurement signal according to a predetermined relationship to derive said second porosity measurement.   
     
     
       47. An apparatus for investigating the porosity of a subsurface earth formation surrounding a borehole, comprising: (a) a sonde adapted to be moved through the borehole; accelerator neutron source means in the sonde for repetitively irradiating the well borehole and surrounding earth formation with bursts of high energy neutrons, which neutrons interact with nuclei of the materials in the borehole and formation to produce therein populations of epithermal neutrons;   (b) a first epithermal neutron detector in the sonde spaced from but close to said neutron source, without substantial intervening high density shielding;   (c) first shielding means for shielding said first detector so as to increase the low energy neutron detection threshold thereof to at least approximately 10 eV;   (d) a second epithermal neutron detector in the sonde located farther from said neutron source than said first detector and at a distance that is the matrix density neutral distance, said second detector being eccentered towards one side of the sonde;   (e) a third epithermal neutron detector in the sonde located intermediate to said first and second detectors relative to said neutron source, said third detector being eccentered towards said one side of the sonde;   (f) third shielding means for shielding said third detector from neutrons incident thereon from all sides thereof except said one side of the sonde;   (g) means for eccentering the sonde in the borehole so that said one side of the sonde is closely adjacent to the borehole wall;   (h) means for separately counting the neutrons detected by said first and second detectors over a time interval encompassing a plurality of said neutron bursts and for generating first and second count signals representative thereof;   (i) means for deriving from said first and second count signals a first measurement correlatable with the porosity of said earth formation;   (j) means for counting the neutrons detected by said third detector during a plurality of time intervals and generating a corresponding plurality of count signals representative thereof;   (k) means for deriving from said plurality of count signals a measurement of the epithermal neutron slowing down time of the earth formation as a second measurement correlatable with the porosity of said formation; and   (l) means for combining said first and second porosity-correlatable measurements to provide an improved measurement of formation porosity.   
     
     
       48. The apparatus of claim 47 wherein said first measurement comprises a ratio of said first and second count signals. 
     
     
       49. The apparatus of claim 47 wherein: said means for deriving said first porosity-correlatable measurement comprises means for deriving a first quantitative measurement of formation porosity; and   said means for deriving said second porosity-correlatable measurement comprises means for deriving a second quantitative measurement of formation porosity.   
     
     
       50. The apparatus of claim 49 wherein said combining said first and second quantitative measurements to derive an improved formation porosity measurement.